Plasmas in Saturn's magnetosphere

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The solar wind plasma analyzer on board Pioneer 11 provides first observations of low‐energy positive ions in the magnetosphere of Saturn. Measurable intensities of ions within the energy per unit charge (E/Q) range 100 eV to 8 keV are present over the planetocentric radial distance range ∼4–16 Rs in the dayside magnetosphere. The plasmas are found to be rigidly corotating with the planet out to distances of at least 10 Rs. At radial distances beyond 10 Rs, the bulk flows appear to be in the corotation direction but with lesser speeds than those expected from rigid corotation. At radial distances beyond the orbit of Rhea at 8.8 Rs, the dominant ions are most likely protons, and the corresponding typical densities and temperatures are 0.5 cm−3 and 106 °K, respectively, with substantial fluctuations. Identification of the mass per unit charge (M/Q) of the dominant ion species is possible in certain regions of Saturn’s magnetosphere via the angular distributions of positive ions. A large torus of oxygen ions is located inside the orbit of Rhea, and the densities are >10 cm−3 over the radial distance range ∼4–7.5 Rs. Density maxima appear at the orbits of Dione and Tethys where oxygen ion densities are ∼50 cm−3. The dominant oxygen charge states are O2+ and O3+ in the radial distance ranges ∼4–7 Rs and 7–8 Rs, respectively. The observations are suggestive of a decrease of ion energies to values less than the instrument energy threshold of E/Q=100 eV at the apparent inward edge of the torus at 4 Rs. Ion temperatures increase rapidly from ∼2 × 105 °K at 4 Rs to 5 × 106 °K at 7.3 Rs. It is concluded that the most likely source of these plasmas is the photodissociation of water frost on the surface of the ring material with subsequent ionization of the products and radially outward diffusion. The sources associated with the satellites Dione and Tethys are probably of lesser strength. The presence of this plasma torus is expected to have a large influence on the dynamics of Saturn's magnetosphere, since the pressure ratio β of these plasmas approaches unity at radial distances as close to the planet as 6.5 Rs. On the basis of these observational evidences it is anticipated that quasi‐periodic outward flows of plasma, accompanied by a reconfiguration of the magnetosphere beyond ∼6.5 Rs, will occur in the local night sector in order to relieve the plasma pressure from accretion of plasma from the rings.